1. Field of the Invention
The invention relates to a roller transmission and gearing mechanism that comprises a driving body, roller means having respective centres or central axes, and a driven body, wherein the driving body is coupled to the driven body by means of the roller means, the driving and driven body are guided for movement having a single degree of freedom, the driving and driven bodies both define respective roller guide tracks thereon, the tracks contact the roller means and determine the relative movement of the roller means with respect to the associated body, the roller means contact the roller guide tracks along respective rolling curves, the roller guide tracks start and terminate on the driving and driven bodies at respective pairs of limit surfaces, the roller means move along their associated roller guide tracks, wherein the roles of the driving and driven bodies can be interchanged.
2. Description of Related Art
Power transmission and gearing systems are fundamental to the mechanical engineering industry and there are a large variety of them available. Most of them can be characterised by among other things their gearing ratio, the maximum transmissible power, their structural design and dimensions, in particular, the relative position and size of the driving and driven bodies, the changeability of the direction of rotation of the driven body relative to the driving body, and last but not least their power transmission efficiency.
Worm gears for example are notorious for their particularly low power transmission efficiency. They dissipate significant amounts of energy due to large frictional energy losses as a result of extensive sliding between their contact surfaces. There have been various propositions made in the past to reduce the extent of sliding via the introduction of rolling balls between the contact surfaces of the grooves of the worm and the teeth of the worm wheel. Thus the worm and the worm wheel were not in direct contact any more but the coupling between them was established via a set of rolling balls. The balls while in coupling position moved along a path between the worm and the worm wheel. When they reached the end of the path they exited the path and disengaged from coupling. Then they were led back through an external device to the beginning of the path where they re-established coupling again.
Such propositions can be found e.g. in U.S. Pat. Nos. 3,365,974, 2,664,760, 4,656,884 and 4,283,329. These designs, however, have not managed to bring on all the benefits that could have been expected from the application of rolling balls because the conditions for pure rolling motion for the balls were not met. In the absence of these conditions the balls were forced to slide extensively along their tracks resulting in sub-optimal transmission efficiency due to significant frictional energy losses.
In the case of classical ball screws that are used for linear movements, for example those used for table movements in machine tools, the conditions for pure rolling motion for the rollers are automatically satisfied. Such a design is shown e.g. in FIG. 3 of U.S. Pat. No. 6,092,434. This is because the driving and driven bodies have collinear or common rotational axes and the roller guide tracks are concentric. In terms of pure rolling motion for the rollers this is the only known example for a roller transmission and gearing system that is used widely for its small frictional energy losses and other advantages of pure rolling motion. In such a motion both the driving and driven bodies are provided by respective roller guide tracks and the balls roll along these guide track and contact the tracks along respective rolling curves. In every moment each ball contacts one contact point of the rolling curve of the driving body and one point of the driven body. As the rolling curves are co-axial helical lines (spirals), the distance between them is constant. The distance between any point of the rolling curve on the driving body with the rolling curve on the driven body can be determined as it is known from the rules of geometry, i.e. if we connect said point with different points of the rolling curve on the driven body, and by definition the shortest one of these connecting lines will be the distance. For coaxial rolling curves this distance will be the same for all points of both rolling curves. These conditions are not fulfilled in case the rolling curves are not coaxial and have forms different from the regular helical lines.
The fact, however, that ball-screws of the mentioned coaxial type can only be applied for transforming rotational movement to a linear displacement along a path parallel to the axis of rotation, makes them inapplicable in providing movements of higher degrees of freedom such as those along two- or three-dimensional paths; and due to such limitations this particular gearing system has not become more generally applicable and widespread.
The most widespread gearing mechanism is trivially the one using toothed wheels. It has numerous advantages and also quite a few disadvantages. One of the disadvantages is that the engagement factor i.e. the number of teeth in simultaneous contact at any given time is relatively small and cannot be increased significantly. This means that the mechanical load is concentrated on a small number of engaging teeth and therefore the maximum transmissible power is relatively low compared to the size and space used, and difficult to increase significantly. Besides the limitations for the maximum transmissible power, there is not much room for manoeuvring to vary the distance and the angle between the driving and driven shafts either. Another constraint for the design is that for a given structural configuration of connecting wheels the relative direction of rotation for the wheels is predetermined. In order to change the relative direction of rotation an extra wheel must be inserted among the wheels. This, on one hand, increases the size of the set-up and, on the other hand, introduces extra frictional energy losses. One of the most important disadvantages of the toothed wheels is frictional energy losses arising from the fact that the connecting teeth of the wheels slide on each other most of the time of their engagement. This causes a significant reduction in the power transmission efficiency even if proper lubrication is applied.